The present invention relates to an image sensing apparatus and method and, more particularly, to an image sensing apparatus and method capable of speeding up and effectively performing an operation for obtaining a single image with a wide dynamic range by processing a plurality of images obtained by sensing an identical scene a plurality of times in different exposures.
Some image sensing apparatuses, such as a video camera, a still video camera, and an electrophotographic camera, use a solid-state electric image sensor, such as a CCD, which generates image signals representing a sensed image. The dynamic range of the solid-state electric image sensing device for incoming light is much narrower than that of silver halide film and the like. Therefore, when a scene to be sensed includes a bright part and a dark part and luminosity difference between them is large, it is very difficult to sense both part in an exposure proper for the solid-state electric image sensing device in a single image sensing operation. In most cases, if the exposure for sensing the scene is adjusted to the dark part of the scene, then the bright part may be expressed in white, and if the exposure is adjusted to the bright part, then the dark part may be expressed in black, for example.
In order to overcome the aforesaid problems, as a method of expanding the dynamic range of a solid-state image sensing device, a technique is proposed in e.g., Japanese Patent Laid-Open No. 63-306778, in which an identical scene is sensed in different exposures to obtain image data of a plurality of sensed images, and the image data of the sensed images is processed in accordance with some predetermined operation to obtain a single image with an expanded dynamic range. Further, Japanese Patent Laid-Open No. 5-64075, titled as "Video Camera, Image Sensing Method Using the Video Camera, and Method of Operating the Video Camera, and Image Sensing Apparatus and Method", proposes a camera and a method for expanding a dynamic range, which senses an identical scene in an exposure that is proper to a dark area of the scene to obtain the first image and in an exposure that is proper to a bright area to obtain the second image, then replaces the image data of the bright area in the first image by the corresponding area in the second image so as to obtain a composite image, and obtains a single image including the dark and bright areas both sensed in the proper exposures.
However, in the aforesaid camera capable of expanding its dynamic range, there were following problems, namely,
I. Two images, i.e., the first image obtained when an exposure is adjusted to the dark part of the scene to be sensed and the second image obtained when an exposure is adjusted to the bright part of the scene, are always sensed. Therefore, when a user does not want dynamic range expansion, another image has to be sensed with an exposure adjusted in accordance with the brightness of the entire scene (the image sensed as this is called "standard image", and the exposure adjusted as this is called as "proper exposure", hereinafter), which is troublesome and operation time for sensing an image is long, and a user may miss a shutter chance.
II. If an electrophotographic camera does not have a photometer, the camera has to perform an image sensing operation (called as "pre-sensing operation", hereinafter) just for deciding an exposure for the subsequent image sensing operation. Then, after a proper exposure for obtaining a standard image is determined on the basis of image data obtained in the pre-sensing operation and the used exposure, the second image sensing operation is performed in the proper exposure (called as "main sensing operation", hereinafter). In the above-described image sensing operation, the image data obtained in the pre-sensing operation is used just for determining an exposure to be used in the subsequent image sensing operation, and abandoned thereafter.
In addition, in order to process a plurality of images to obtain a single image with the expanded dynamic range in accordance with the image sensing processing described in the previous paragraph, in order to obtain the plurality of images, it is necessary to determine the proper exposure by performing the pre-sensing operation, then to perform main image sensing operation in the proper, over, and under exposure.
However, as described above, image data obtained in the pre-sensing operation is conventionally used only for photometry, and abandoned thereafter. Consequently, it is necessary to perform image sensing operation at least three times for obtaining a plurality of images (at least two images) in different exposures, which is troublesome and consumes a long time.
Furthermore, since an image is sensed in the pre-sensing operation, and more than two images are sensed in the main sensing operation, there is a time gap between when an image is sensed for the first time and when an image is sensed in the n-th time. Accordingly, a problem arises that images to be processed may be considerably different from each other.
III. Since it is necessary to store a plurality of sensed images to perform the aforesaid dynamic range expansion processing, there is a problem that an amount of image data to be stored is quite large.